arXiv subjects

Jay Sau

Publications and source records attributed to Jay Sau.

6 recordsLinked to original sources

arXiv2024

Dynamically Robust Counterdiabatic Topological Pumping

Thouless pumping is a transport phenomenon whereby an insulating, time-periodic Hamiltonian induces a robustly quantized transfer of charge, per driving cycle, in the quasi-adiabatic limit. In this work we apply the shortcuts to adiabaticity (STA) method of counterdiabatic (CD) driving to the Rice-Mele model, an archetypal model for Thouless pumping. We show that the charge pumped rapidly across each bond of our CD Rice-Mele model is robustly quantized and determined by a Chern number. The CD Hamiltonian generally involves long-range, hence non-local, hopping. Therefore we also derive an exact, local, nearest-neighbor CD Hamiltonian for our lattice system. We show that our nearest neighbor non-adiabatic protocol possesses the same topological robustness to onsite disorder as quasi-adiabatic Thouless pumps and is moreover surprisingly robust to onsite disorder and noise errors in implementing the CD protocol, for sufficiently rapid driving. The protocol also reproduces finite-temperature Thouless pump results away from the adiabatic limit. In addition, we develop a general method to produce non-trivial, finite-time quantized pumping starting from any Rice-Mele initial ground state using only nearest-neighbor hopping.

quant-ph↗

arXiv2024

Proposal for Majorana Modes without a Magnetic Field in a semiconductor-superconductor sandwich structures

We propose a planar Josephson junction setup to host Majorana modes where the semiconductor is sandwiched between superconductors on both surfaces. While the studied configuration is related to both devices proposed for use with topological insulators as well as other proposals for Majorana modes without applied magnetic fields, we find that placing the superconductor on both surfaces allows us to replace the topological insulator by a wider variety of spin-orbit coupled semiconductors. Application of an electric field in the junction together with appropriate superconducting phase differences allows different Fermi surfaces to be subject to different phase differences. We find that these conditions can drive the system into a class-D topological superconductor with a pair of Majorana modes at the two ends of the junction. Additionally, class-DIII topological superconductors with helical Majorana modes also occur in other parts of the phase diagram. We simulate our setup on HgTe near the valence band edge described by the well-known 8-band Kane model with realistic parameters and find phase diagram can be achieved. The topological gap can remain comparable to the bulk superconducting gap throughout the fermi surface.

cond-mat.supr-con↗

arXiv2023

Optical pumping of electronic quantum Hall states with vortex light

A fundamental requirement for quantum technologies is the ability to coherently control the interaction between electrons and photons. However, in many scenarios involving the interaction between light and matter, the exchange of linear or angular momentum between electrons and photons is not feasible, a condition known as the dipole-approximation limit. An example of a case beyond this limit that has remained experimentally elusive is when the interplay between chiral electrons and vortex light is considered, where the orbital angular momentum of light can be transferred to electrons. Here, we present a novel mechanism for such an orbital angular momentum transfer from optical vortex beams to electronic quantum Hall states. Specifically, we identify a robust contribution to the radial photocurrent, in an annular graphene sample within the quantum Hall regime, that depends on the vorticity of light. This phenomenon can be interpreted as an optical pumping scheme, where the angular momentum of photons is transferred to electrons, generating a radial current, and the current direction is determined by the vorticity of the light. Our findings offer fundamental insights into the optical probing and manipulation of quantum coherence, with wide-ranging implications for advancing quantum coherent optoelectronics.

cond-mat.mes-hall↗

arXiv2021

From Majorana fermions to topological quantum computation in semiconductor/superconductor heterostructures

We present a pedagogical review of topological superconductivity and its consequences in spin-orbit coupled semiconductor/superconductor heterostructures. We start by reviewing the historical origins of the notions of Dirac and Majorana fermions in particle physics and discuss how lower dimensional versions of these emerge in one dimensional superconductors. Ultimately, we focus on Majorana zero-modes, which emerge at defects in the Majorana equation. We then review the definition of the topological invariant, and how it allows the prediction of such Majorana modes from the bulk bandstructure of realistic superconductors, which do not have Lorentz invariance. Finally, we end with a discussion of protocols for how to detect such Majorana modes and use them for topological quantum computation.

cond-mat.supr-con↗

arXiv2021

A classical model for sub-Planckian thermal diffusivity in complex crystals

Measurements of thermal diffusivity in several insulators have been shown to reach a Planckian bound on thermal transport that can be thought of as the limit of validity of semiclassical phonon scattering. Beyond this regime, the heat transport must be understood in terms of incoherent motion of the atoms under strongly anharmonic interactions. In this work, we propose a model for heat transport in a strongly anharmonic system where the thermal diffusivity can be lower than the Planckian thermal diffusivity bound. Similar to the materials which exhibit thermal diffusivity close to this bound, our scenario involves complex unit cell with incoherent intra-cell dynamics. We derive a general formalism to compute thermal conductivity in such cases with anharmonic intra-cell dynamics coupled to nearly harmonic inter-cell coupling. Through direct numerical simulation of the non-linear unit cell motion, we explicitly show that our model allows sub-Planckian thermal diffusivity. We find that the propagator of the acoustic phonons becomes incoherent throughout most of the Brillouin zone in this limit. We expect these features to apply to more realistic models of complex insulators showing sub-Planckian thermal diffusivity, suggesting a multi-species generalization of the thermal diffusivity bound that is similar to the viscosity bound in fluids.

cond-mat.mtrl-sci↗

arXiv2021

From anyons to Majoranas

Anyons, particles that are neither bosons nor fermions, were predicted in the 1980s, but strong experimental evidence for the existence of the simplest type on anyons has only emerged this year. Further theoretical and experimental advances promise to nail the existence of more exotic types on anyons, such as Majorana fermions, which would make topological quantum computation possible.

cond-mat.supr-con↗